Intelligent Electronic Lock

This intelligent electronic lock circuit is built using transistors only. To open this electronic lock, one has to press tactile switches S1 through S4 sequentially. For deception you may annotate these switches with different numbers on the control panel/keypad. For example, if you want to use ten switches on the keypad marked ‘0’ through ‘9’, use any four arbitrary numbers out of these for switches S1 through S4, and the remaining six numbers...

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Luxurious Toilet Bathroom Facility

Aged persons in the house and guests often fumble while searching for the toilet and bathroom switches at night. Also, very few of us take care to switch off the lights of toilets/bathrooms after using them. The circuit given here helps to overcome both the problems. The figure shows two symmetrical circuits (one each for toilet and bathroom) sharing common power supply and a melody generator-cum-audio warning unit. The reed switches S1 and...

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Automatic Heat Detector

This circuit uses a complementary pair comprising npn metallic transistor T1 (BC109) and pnp germanium transistor T2 (AC188) to detect heat (due to outbreak of fire, etc) in the vicinity and energise a siren. The collector of transistor T1 is connected to the base of transistor T2, while the collector of transistor T2 is connected to relay RL1. The second part of the circuit comprises popular IC UM3561 (a siren and machine-gun sound generator...

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Musical Touch Bell

Here is a musical call bell that can be operated by just bridging the gap between the touchplates with one’s fingertips. Thus there is no need for a mechanical ‘on’/‘off’ switch because the touch-plates act as a switch. Other features include low cost and low power consumption. The bell can work on 1.5V or 3V, using one or two pencil cells, and can be used in homes and offices. Two transistors are used for sensing the finger touch and switching...

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High Power Bicycle Horn

An interesting circuit of a bicycle horn based on a popular, low cost telecom ringer chip is described here. This circuit can be powered using the bicycle dynamo supply and does not require batteries, which need to be replaced frequently. The section comprising diodes (D1 and D2) and capacitors (C1 and C2) forms a half-wave voltage-doubler circuit. The output of the voltage doubler is fed to capacitor C3 via resistor R1. The maximum DC supply...

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Overheat Detector Alarm/Switch

At the heart of this circuit is a precision integrated temperature sensor type LM35 (IC1), which provides an accurately linear and directly proportional output in mV, over the zero to +155 degrees C temperature range. The LM35 develops an output voltage of 10 mV/K change in measured temperature. Designed to draw a minimal current of its own, the LM35 has very low self heating in still air. Here the output of the LM35 is applied to the non-inverting...

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Test Beeper For Your Stereo

The test beeper generates a sinusoidal signal with a frequency of 1,000 Hz, a common test frequency for audio amplifiers. It consists of a classical Wien-Bridge oscillator (also known as a Wien-Robinson oscillator). The network that determines the frequency consists here of a series connection of a resistor and capacitor (R1/C1) and a parallel connection (R2/C2), where the values of the resistors and capacitors are equal to each other. This...

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Stepped Volume Control

Louder music, sirens or speech in response to higher ambient noise levels? This simple circuit has the answer, and it may enable your robot to be at least as noisy or loud-mouthed as the others in an arena. The circuit consists basically of a microphone, a level detector, a 4-state counter and four analogue switches connected to a resistive ladder network. Looking at the circuit diagram, the signal from electret microphone M1 is amplified by...

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Converting a DCM Motor

We recently bought a train set made by a renowned company and just couldn’t resist looking inside the locomotive. Although it did have an electronic decoder, the DCM motor was already available 35 (!) years ago. It is most likely that this motor is used due to financial constraints, because Märklin (as you probably guessed) also has a modern 5-pole motor as part of its range. Incidentally, they have recently introduced a brushless model. The...

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DRM Direct Mixer Using an EF95/6AK5

This hybrid DRM receiver with a single valve and a single transistor features good large-signal stability. The EP95 (US equivalent: 6AK5) acts as a mixer, with the oscillator signal being injected via the screen grid. The crystal oscillator is built around a single transistor. The entire circuit operates from a 6-V supply. The receiver achieves a signal-to-noise ratio of up to 24 dB for DRM signals. That means the valve can hold its own against...

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Negative-Output Switching Regulator

There are only a limited number of switching regulators designed to generate negative output voltages. In many cases, it’s thus necessary to use a switching regulator that was actually designed for a positive voltage in a modified circuit configuration that makes it suitable for generating a negative output voltage. The circuit shown in Figure 1 uses the familiar LM2575 step-down regulator from National Semiconductor (www.national.com). This...

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Two-Cell LED Torch

It sometimes comes as a bit of a shock the first time you need to replace the batteries in an LED torch and find that they are not the usual supermarket grade alkaline batteries but in fact expensive Lithium cells. The torch may have been a give away at an advertising promo but now you discover that the cost of a replacement battery is more than the torch is worth. Before you consign the torch to the waste bin take a look at this circuit. It...

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Temperature Reference

It is often difficult to properly calibrate a temperature sensor since there is no suitable aid for doing so available. This article, which describes a temperature reference source, aims at putting this right. Since the source is made variable, the reference temperature may also be used for adjusting thermostats correctly. This may prove useful in the case of the recently published Titan 2000 audio power amplifier. The diagram shows how a Type...

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Mains Voltage Detector

The detector is intended to sense and signal to another circuit that an appliance is connected to the mains voltage. For this purpose, an optocoupler, IC1 in the circuit, is used. The light-emitting diode in this device is connected across the mains voltage rectified by bridge B1. The mains voltage is applied to this bridge via potential divider R1-C1-R2. When the capacitor has a value as specified in the diagram, the current through the diode...

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Single-Supply Instrumentation Amplifier

The OP284 is a low noise dual op amp with a bandwidth of 4MHz and rail-to-rail input/output operation. These properties make it ideal for low supply voltage applications such as in a two op amp instrumentation amplifier as shown in the diagram. The circuit uses the classic two op amp instrumentation topology with four resistors to set the gain. The transfer equation of the circuit is identical to that of a non-inverting amplifier. Resistors...

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+/– Voltage On Bargraph Display

The LM3914 is a truly versatile component. Besides LEDs, only a few other components are needed to make the ‘bidirectional’ bargraph voltmeter shown here. The circuit is similar to a conventional bar display, but it offers a possibility to change the direction in which the LEDs are switched on. This may be useful, for example, when positive and negative voltages are measured. For a positive input voltage, the LEDs are switched on in the usual...

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PC Fan Speed Controller - For a Low-Noise PC

The fan runs constantly in many PCs, which may not even be necessary. A simple controller circuit can regulate the fan speed according to demand. This not only saves energy, it also reduces irritation from the fan noise. Only three components are needed to allow the fan speed to be controlled according to the actual demand: one adjustable voltage regulator and two resistors that form a voltage divider. One of the resistors is a NTC thermistor...

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Rugged PSU For Ham Radio Transceivers

This rugged power supply is based on the popular LM338 3-pin voltage regulator. The LM338 is capable of supplying 5 A over an output voltage range of 1.2 V to 32 V with all standard protections like overload, thermal shutdown, over-current, internal limit, etc., built in. In this power supply, some extra protections have been added to make it particularly suitable for use with low to medium-power portable and mobile VHF/UHF (ham) and 27 MHz...

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Power Diode For Solar Power Systems

Apart from the sun, solar power systems cannot work without a reflow protection diode between the solar panel and the energy store. When current flows into the store, there is a potential drop across the diode which must be written off as a loss in energy. In the case of a Schottky diode, this is not less than 0.28 V at nominal current levels, but will rise with higher ones. It is clear that it is advantageous to keep the energy loss as small...

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General-Purpose NiCd Battery Charger

There is a wide variety of NiCd (nickel-cadmium) battery chargers on the market, but there are not many that can work from an in car 12 V cigar lighter. Such a charger would, for instance, be of interest to campers and caravanners who do not have a 230 V a.c. mains supply available. To satisfy the needs of these users, a charger could be designed for operation from the cigar lighter, but it is, of course, of far greater interest if it could...

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Absolute-Value Meter With Polarity Detector

This circuit breaks an input voltage signal down into its components: (1) the absolute value and (2) the polarity or ‘sign’ (+ or –). It will handle direct input voltages as well as alternating voltages up to several kHz. With a supply voltage of ±9 V, the input level should remain below ±6V. The circuit consists of two sections, each having its own function. Operational amplifiers IC1a and IC1b form a full-wave rectifier, its output terminal...

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Multiple Continuity Tester

The continuity tester is a handy adjunct to an ohmmeter. The unit or component whose continuity is to be checked is connected between terminals E1 and E2 (which may be probes or croc clips). The test current then flowing through the unit/component on test causes a potential drop across resistor R2, which is applied to the non-inverting input of buffer IC2. The output of the op amp is applied to transistor T1, in the emitter circuit of which...

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